Two of the largest engines of the world economy -- United States and China -- have slowed down.

China simply wasn't able to sustain its incredible growth, and that has had ripple effects around the world. Manufacturing and exports have cooled, and the real estate market isn't the slam dunk that it once was.

In the U.S., the strong dollar has been a drag on growth. American companies are losing money overseas and foreigners aren't buying as many U.S. goods since they appear more expensive.

The OECD slashed its U.S. 2015 growth projection from 3.1% to 2%. If that comes to pass, it would be a dip from last year's 2.4% GDP.

Outlook for America: The U.S. was supposed to lead the way this year, but its economy contracted in the first quarter. Experts say that was a blip and growth will pick up soon. The question is whether the bounce back will be strong enough to lift the American economy to a better year than last year.

In a good sign for workers, hiring has remained strong in the U.S., but people and businesses just aren't spending much money yet. That has constrained growth.

"We've seen real income go up and savings increase, but consumer spending has not improved as much as we expected it to," says Robin Anderson, senior economist at Principal Global Investors. "But we are already starting to see the rebound in the second quarter data."

The projection for 2016 has been cut from 3.0% in March to 2.8% in the OECD report.

"Most economists, including myself, expected that the economy would be normalized by 2015 but we have had to push out our expectations to 2016," says Tim Hopper, chief economist at TIAA-CREF Financial Services.

Outlook for the world: Much of the rest of the world looks equally shaky, if not worse.

If Greece fails to reach an agreement with its creditors, Europe will feel the strain again, especially in business confidence. While Germany's economy remains the lynchpin, overall unemployment is 11.2% in the euro area.

The OECD continues to call for more structural reforms in Europe as well as better access to credit in some countries.

As one of the world's largest economies, China's expected decline in growth in 2015 and 2016 also raises red flags. China's government is reacting quickly with more plans to step up infrastructure spending. Much like in the U.S., the key to getting out of the rough patch will be whether China's growing middle and upper classes keep spending.

The OECD says the global economy could easily get an A grade again, but it will take a lot more efforts from governments and central banks around the world to boost growth.